1. Field of the Invention
The invention relates to high speed cable links, and in particular, to adaptive data reception for signals transferred through a cable.
2. Description of the Related Art
FIG. 1 shows an eye diagram. Cables or conductive wires are widely used media for signal transmission, in which digital data are sequentially transmitted in symbol form. A symbol may be an application dependent waveform having predetermined amplitude and duration. For example, in FIG. 1, a symbol generated at the transmitter end may have amplitude VS and data period TP. During the data period TP, a duration between time points ta and tb is referred to as the hold time TH, whereas the others setup time TS. A valid symbol value is only acquirable during the hold time TH where the symbol amplitude is kept at a desirable level. For a high speed cable link, the symbol is transmitted through a cable and sampled at a receiver end. Ideally, the symbol received at the receiver end remains a proper waveform so that the original output value can still be correctly acquired by sampling the symbol within the hold time TH. Practically, however, a cable is equivalent to a low pass filter that reduces signal quality. Not only is amplitude decayed but time delay is induced to different extents at different frequency bands. Thus, a distorted symbol may cause a so-called data jitter, reducing the possibility to correctly sample an output value. Since the transition edges in a non-ideal cable are uncertain, the available hold time where correctness is ensured, is shortened. For example, the duration between time points ta′ and tb′ represents the hold time TH′ being shortened due to data jitter. As the available hold time TH′ is shortened, duration of uncertainty is increased and implementation of an accurate sampling mechanism becomes more difficult.
FIG. 2 shows a conventional data receiver 200 at the receiving end of a cable. The data extractor 204 is designed to acquire output values #DOUT from the input signal #DIN that has passed through the cable (not shown). The input signal #DIN may be distorted when passing through the cable, having data jitter that is difficult to reacquire output values therefrom. Thus, an equalizer 202 may be provided to equalize the input signal #DIN before it is sent to the data extractor 204. The equalization is a mathematical function that reverses the distortion effects induced by the cable, and the equalizer 202 is typically configured with a fixed boost value as a parameter for the mathematical function, thereby the distorted input signal #DIN can be recovered from an equalized signal #DQ having quality subsequently approximating to the original symbol. In this way, the correctness of output value #DOUT obtained by the data extractor 204 can be efficiently improved.
The distortion caused by different cables may vary. For example, the input signal #DIN may feature different characteristics dependent on the frequencies, cable lengths and cable materials. Thus, if an equalizer 202 with fixed boost value is applied for unknown cable feature, under equalization or over equalization may consequently occur. Under equalization, in particular, often causes severe data jitter that greatly impacts efficiency of signal transmission. Therefore, an enhancement for the equalizer 202 is desirable.